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In its broadest sense, the environment can be defined as external conditions influencing the development of people, animals or plants. For the purpose of studying the effects of the environment on human health, a distinction is often made between conditions from which individuals may have no or only partial control and those for which some element of personal choice exists. Exposure encountered at work and to substances in air and water would, for example, tend to fall in the former category, while “lifestyle” factors such as smoking, eating a high fat diet, and drinking alcohol would come in the latter. Higginson1 points out that, in the public mind “environmental cancer” is a term often limited to cancers resulting from chemical exposures, especially manmade, although most research workers use it in the wider sense to cover all conditions that impact on human cancer.
This paper gives an overview of environmental causes of cancer and the approaches used in the investigations of this issue, discusses the controversies and challenges, and outlines some of the emerging scientific methodology.
CARCINOGENICITY
The International Agency for Research into Cancer (IARC) classifies carcinogenic substances into four groups according to the evidence (table 1). For human data, sufficient evidence is defined as the establishment of a causal relation between exposure to the agent and human cancer. Limited evidence is defined as the observation of a positive association between exposure to the agent and human cancer, for which a causal interpretation is considered credible, but chance, bias or confounding could not be ruled out with reasonable confidence. Similar definitions relate to the evidence from experimental data (see the IARC monographs on the evaluation of carcinogenic risks to humans for a full description, for example2).
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Carcinogenicity defined by the International Agency for Research into Cancer
In the USA, …